Tired of motion-sensing bathroom lights? There's hope!

So there you are; needing to use the bathroom. The room is dark and there’s no light switch. One of those motion sensors is on the wall. It’s supposed to detect you moving into the room and then turns the light on. Only it doesn’t. You move in and jump around, always holding the door open with one hand so you are not left totally in the dark. Finally the light goes on and you breathe a sigh of relief.

Sound familiar? Now comes news the bathroom dance may no longer be needed.

In a pair of studies, researchers at Duke University and Institut Lengevin, France have shown that patterns made by radio waves can detect a person’s presence and location anywhere in a room. It could lead to a new motion sensor based on metamaterials that are sensitive enough to monitor a person’s breathing.

"Energy companies don’t love infrared motion detectors because they have lots of problems,” said David R. Smith, the James B. Duke Professor of Electrical and Computer Engineering at Duke. “The detectors cover a limited amount of space, a person has to be within their line of sight to be detected, and probably everyone has had the experience where the lights have gone off because they’ve sat still for too long. Radio waves can get around all of these limitations.”

Using radio wave behavior to sense motion

The new technology works by taking advantage of the way radio waves behave in an enclosed room. The waves continuously reflect off multiple surfaces, creating complex interference patterns throughout the room. These unique patterns change when anything in the room changes. That means a sensitive antenna could detect when something moves in or enters the room.

“The complexity of the way radio waves bounce around a room and interfere with themselves creates a sort of fingerprint,” explained Philipp del Hougne, a researcher visiting Smith’s laboratory from Institut Langevin in Paris, France. “And each time an object within a room moves, even a little bit, that fingerprint changes."

The challenge lies with finding the most efficient way for a detector to learn that radio wave fingerprint in the first place. Researchers propose using metamaterials; artificial materials that can manipulate light and sound because of the properties of their structure. Metamaterials would let detectors control the shape of the waves and make it easier to detect differences, such as when a person enters a room.

Turning on the bathroom light might be the most practical application, but there are also applications in smart homes for energy savings and security as well as healthcare. The findings appeared recently in Scientific Reports and Aug. 6 in the Physical Review Letters.

—Frank Graff

Frank Graff is a producer/reporter with UNC-TV, focusing on Sci Tech Now North Carolina, a weekly science series that airs Tuesdays on UNC-TV. In addition to producing these special segments, Frank will provide additional information related to his stories through this North Carolina Science Now Reporter's Blog!